1. Field of the Invention
The present invention relates to a selective-calling radio receiver such as a pager and more particularly, to a selective-calling radio receiver capable of receipt of signals in two or more different frequency bands.
2. Description of the Prior Art
In recent years, the parasitic capacitance of transistors has been decreased according to miniaturization of electronic elements and components integrated on Integrated circuits (ICs), thereby raising the transition frequency fT of the transistors. Under such the circumstances, the operating frequency of various circuits incorporated into ICs has been becoming higher.
A typical one of the selective-calling radio receivers is a portable receiver called a xe2x80x9cpagerxe2x80x9d or xe2x80x9cpaging receiverxe2x80x9d. The paging receiver is usually equipped with a frequency synthesizer using a Phase-Locked Loop (PLL) circuit (i.e., a PLL frequency synthesizer) as a local oscillator or a carrier wave generator. Since a receiver circuit implemented in the conventional paging receiver needs to operate at a voltage as low as 1 V supplied by a dry battery, various PLL circuits capable of operation at a low voltage such as 1 V with low power dissipation have been developed and actually used for the PLL frequency synthesizer. A main part of the FLL circuit excluding a Voltage-Controlled Oscillator (VCO) and a low-pass filter is usually formed on an IC chip, which has been termed a xe2x80x9cPLL ICxe2x80x9d.
Several years ago, the highest operating frequency of the PLL IC at a voltage of 1 V was approximately 100 MHz. However, recently, this highest operating frequency has been raised to approximately 200 to 300 MHz and at the same time, the power dissipation has become negligible in view of the receiver operation. This tendency seems to progress further in the future.
The increase of the operating frequency of the PLL IC means that the output frequency of the VCO is increased. Therefore, there is an advantage that the configuration of the PLL circuit is simplified. For example, a frequency multiplier or multipliers incorporated into the PLL IC may be unnecessary at specific frequencies, or the number of the frequency multiplication operations in the PLL IC maybe decreased. In this case, the carrier-to-noise ratio (C/N) degradation in the paging receiver is suppressed, which improves the performance or characteristics of the paging receiver.
Conventionally, the paging receiver typically uses the well-known xe2x80x9cdirect conversion methodxe2x80x9d. In this method, the frequency of a received signal is directly converted to its baseband frequency without using any Intermediate Frequency (IF). Also, there is an advantage that an external filter is unnecessary to simplify the circuit configuration.
With the paging receiver using the direct conversion method, circuit parameter changes are required according to the frequency band of the received signal in (i) the VCO and the phase shifter in the local or carrier-wave oscillator and (ii) circuits from an antenna to a Radio-Frequency (RF) amplifier.
Therefore, if electronic components or parts relating to the VCO and the phase shifter in the carrier-wave oscillator and the circuits from the antenna to the RF amplifier can be commonly used for different frequency bands of a received signal, it is effective for components/parts management and production control in fabrication of the paging receivers of this sort.
Various prior-art receivers with the intention of common use of the components or parts have been developed and disclosed.
A first one of the prior-art receivers is disclosed in the Japanese Non-Examined Patent Publication No. 56-136041 published in Oct. 1981. This prior-art receiver is comprised of a single VCO provided in a PLL frequency synthesizer. This single VCO is commonly used for different frequency bands of a received signal, such as the Amplitude Modulation (AM) and Short Wave (SW) radio-broadcasting bands.
With the first prior-art receiver disclosed in the Japanese Non-Examined Patent Publication No. 56-136041, the output frequency of the single VCO circuit is multiplied and then, the frequency-multiplied output of the VCO circuit is compared in phase with a reference signal by a phase detector or phase comparator provided in a PLL frequency synthesizer. On the other hand, the output frequency of the single PLL circuit is multiplied by a corresponding one of frequency multipliers to a desired band, thereby forming a local signal of a reference frequency. This local signal is then frequency-mixed with a received signal in a desired one of the AM and SW bands by a frequency multiplier for the desired band.
A frequency-divided output of the single VCO circuit, which is produced by a programmable frequency divider, may be used instead of the output itself of the single VCO circuit.
With the first prior-art receiver disclosed in the Japanese Non-Examined Patent Publication No. 56-136041, the single VCO can be commonly used for the AM and SW bands. However, there is a problem that any other components of the receiver, such as a phase shifter for producing an In-phase carrier signal and a Quadrature-phase carrier signal from a carrier signal in an orthogonal converter, is not commonly used.
A second one of the prior-art receivers is disclosed in the Japanese Non-Examined Patent Publication No. 8-317002 published in Nov. 1996. This prior-art receiver is comprised of a local oscillator for generating a wave of a frequency twice as much as an intermediate frequency (IF), which is provided on a same device of a quadrature modulation circuit. This local oscillator and a phase detector constitute a PLL circuit for generating a signal synchronized with an output clock of a crystal oscillator. The signal generated by the PLL circuit is then frequency-divided by two by a 90xc2x0-phase shifter to thereby produce two carrier waves having a phase difference of 90xc2x0 for quadrature modulation. These two carrier waves are multiplied with inputted I and Q signals by corresponding frequency mixers, respectively.
With the second prior-art receiver disclosed in the Japanese Non-Examined Patent Publication No. 8-317002, a wave of a frequency twice as much as an IF frequency is generated by the local oscillator and then, this wave is frequency-divided by the phase shifter to form the two carrier waves with a phase difference of 90xc2x0. Therefore, no frequency doubling circuit such as the Gilbert multiplier nor band-pass filter are required. Thus, obtainable modulation/demodulation accuracy is improved and no change is required for different frequency bands for portable telephones.
However, an IF signal is used in the second prior-art receiver Therefore, the configuration in this receiver is not applicable to a receiver using the direct conversion method.
A third one of the prior-art receivers is disclosed in the Japanese Non-Examined Patent Publication No. 9-200070 published in Jul. 1997. This prior-art receiver is designed to convert received signals in different frequency bands to a common Intermediate-Frequency (IF) signal, thereby simplifying the circuit configuration.
A received signal is classified by a first switching means according to its frequency band. Then, the received signal is sent to a first frequency mixer through a corresponding one of filters and a corresponding one of amplifiers and a second switching means. The first and second switching means are controlled by a frequency switching signal. On the other hand, a first local signal generated by a first local oscillator is supplied to the first frequency mixer. Thus, a first IF signal of a first IF is produced by mixing the received signal and first local signal in the first frequency mixer.
The first local signal has a first local frequency that corresponds to the frequency band of the received signal. The first local frequency is changed according to change of the frequency band of the received signal in such a way that the output of the first frequency mixer is always equal to the first IF.
The output of the first frequency mixer (i.e., the first IF signal) is sent to a second frequency mixer through a filter. On the other hand, a second local signal of a second local frequency is supplied to the second frequency mixer. Thus, a second IF signal of a second IF is produced by mixing the first IF signal and the second local signal.
With the third prior-art receiver disclosed in the Japanese Non-Examined Patent Publication No. 9-200070, the first local frequency is changed according to the change of the frequency band of the received signal in such a way that the output of the first frequency mixer is always equal to the first IF. Therefore, the subsequent stages to the first frequency mixer in the receiver can be commonly used for different frequency bands of the received signal.
However, similar to the above-described second prior-art receiver, the first and second IF signals are used in the third prior-art receiver. Therefore, the configuration of this receiver is not applicable to a receiver using the direct conversion method.
Accordingly, an object of the present invention is to provide a selective-calling radio receiver using the direct conversion method that makes it possible to commonly use a VCO and its neighboring component for different frequency bands.
Another object of the present invention is to provide a selective-calling radio receiver using the direct conversion method that facilitates the components/parts management and production control in fabrication of the receiver.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
A selective-calling radio receiver according to the present invention is comprised of (a) a PLL frequency synthesizer for generating an initial local signal, the PLL frequency synthesizer comprising a VCO generating the initial local signal and a PLL circuit controlling the initial local signal; (b) an orthogonal converter for orthogonally converting a digitally-modulated received signal to a combination of first and second baseband signals having a phase difference of 90xc2x0 using the initial local signal, the orthogonal converter including (b-1) a first frequency multiplier for multiplying the initial local signal by a variable multiplication factor to produce a multiplied initial local signal, (b-2) a first phase shifter for producing first and second local signals having a phase difference of 90xc2x0 from the multiplied initial local signal, (b-3) a first frequency mixer for mixing the first local signal with the received signal to produce the first baseband signal, and (b-4) a second frequency mixer for mixing the second local signal with the received signal to produce the second baseband signal; (c) an orthogonal-converter controller for controlling a characteristic of the orthogonal converter according to a frequency band of the received signal, the orthogonal-converter controller controlling the orthogonal converter so as to set the variable multiplication factor of the first frequency multiplier as a desired value and to optimize a characteristic of the first phase shifter according to the frequency band of the received signal; (d) a demodulator for demodulating the first and second baseband signals to produce a demodulated signal; and (e) a decoder for decoding the demodulated signal to derive information transmitted by the received signal.
With the selective-calling radio receiver according to the present invention, the orthogonal converter includes the first frequency multiplier for multiplying the initial local signal by the variable multiplication factor to produce the multiplied initial local signal and the first phase shifter for producing the first and second local signals from the multiplied initial local signal.
Moreover, the orthogonal-converter controller is additionally provided to control the characteristic of the orthogonal converter according to the frequency band of the received signal. Specifically, the orthogonal-converter controller controls the orthogonal converter so as to set the multiplication factor of the first frequency multiplier as a desired value and to optimize the characteristic of the first phase shifter according to the frequency band of the received signal.
Accordingly, the VCO of the frequency synthesizer, and the first frequency multiplier and the first phase shifter of the orthogonal converter (i.e., the neighboring components of the VCO) are able to be commonly used for different frequency bands of the received signal. This means that these components or parts are able to be commonly used even if the frequency of the received signal is changed. As a result, the components/parts management and production control in fabrication of this receiver is facilitated by adjusting the characteristic of the orthogonal converter according to the frequency band of the received signal.
In a preferred embodiment of the receiver according to the present invention, the first phase shifter is comprised of a capacitance-variable capacitor whose capacitance is changed by a control signal. In this case, there is an additional advantage that the characteristic of the first phase shifter is readily changed.
In another preferred embodiment of the receiver according to the present invention, a second phase shifter is additionally provided. The first and second local signals are produced by the first and second phase shifters, respectively. Each of the first and second phase shifters is comprised of a capacitance-variable capacitor whose capacitance is changed by a control signal. In this case, there is an additional advantage that the characteristics of the first and second phase shifters are readily optimized.
It is preferred that one of the first and second phase shifters has a configuration of a high-pass filter including a capacitor and a resistor and the other thereof has a configuration of a low-pass filter including a capacitor and a resistor. In this case, there is an additional advantage that the first and second phase shifters are readily configured.
In still another preferred embodiment of the receiver according to the present invention, a second frequency multiplier is additionally provided. One of the first and second frequency multipliers is selectively used according to the frequency band of the received signal. In this case, there is an additional advantage that the changeable frequency range of the multiplied initial local signal becomes wider.
It is preferred that one of the first and second frequency multipliers is selectively activated by supplying/sinking a current to/from the selected one of the first and second frequency multipliers. In this case, there is an additional advantage that the selection of the first and second multipliers is readily carried out.
In a further preferred embodiment of the receiver according to the present invention, a second phase shifter and a second frequency multiplier are additionally provided. The first and second local signals are produced by the first and second phase shifters, respectively. Each of the first and second phase shifters is comprised of a capacitance-variable capacitor whose capacitance is changed by a control voltage supplied from a voltage source. One the first and second frequency multipliers is selectively activated according to the frequency band of the received signal by supplying a current supplied/sunk by a current source/sink to/from a desired one of the first and second frequency multipliers. The voltage source and the current source/sink are formed on an IC chip on which a main part of the PLL frequency synthesizer is formed. In this case, the advantages of the present invention are effectively exhibited.
Preferably, a set of data for the control signal are stored in a rewritable Read-Only Memory (ROM), and they are designed to be read out by a main controller. In this case, there is an additional advantage that the frequency band of the receiver can be readily changed by simply rewriting the content of the rewritable ROM.